feat(companion): rename Tabby companion to Sagi, replace cat avatar with mascot

Full rename across files, identifiers, CSS classes, and user-facing strings
(en/vi locales, ARCHITECTURE.md). Replaces the hand-drawn animated cat
avatar with the pig-superhero mascot artwork, split into parts to keep
per-eye cursor tracking and blink animation working; whole-mascot mood
transforms (breathe/bob/shake/tilt) and new zzz/bang/sparkle overlays
carry the rest of the mood expression since the traced art has no shared
palette to key off of.

scripts/expand-ts-module-docs.py

@@ -73,9 +73,9 @@ def topic_blurb(path: Path) -> str:
             "Renders Claude transcript rows (user, assistant, tool calls) inside Session "
             "Detail with markdown, syntax highlighting, and TUI-style segments."
         )
-    if "Tabby" in rel:
+    if "Sagi" in rel:
         hints.append(
-            "Tabby is the optional on-screen cat assistant — quips, intents, and lightweight "
+            "Sagi is the optional on-screen mascot assistant — quips, intents, and lightweight "
             "event reactions layered above the dashboard chrome."
         )
     if "hook" in rel.lower() or name.startswith("use"):